1. Field of the Invention
This invention relates to an electroplating method and apparatus for producing a length of metal foil, typically copper foil and a split insoluble anode used therein.
2. Prior Art
Printed circuit boards are in widespread use in a variety of fields. The printed circuit boards use copper foil which is commonly produced by electroplating. In the manufacture of electroplated copper foil, it is essential that the foil is free of point defects such as pinholes and anomalous deposits and has a uniform thickness.
In the conventional manufacture of electrolytic copper foil, the cathode is a rotating drum of titanium or stainless steel (SUS) and the anode is a pair of lead plates having an arcuate cross section corresponding to approximately a quarter of the drum circumference. The anode plates are disposed below and concentrically with the cathode drum to define a channel between the cathode drum and the anode and a lower opening or slit between the anode plates. A plating solution is supplied into the channel through the lower slit. Reference to FIG. 8 will help in the understanding of this arrangement. Direct current is conducted between the cathode and anode to deposit copper on the cathode drum. A length of copper foil is continuously separated from the drum and taken up on a roll.
The anode used in the prior art is generally formed of pb or binary or multi-component alloys of pb with Sb, Sn, Ag, In, Ca or the like. Then during electroplating, lead oxide forms on the anode surface and leaches into the electrolytic solution in the form of pb ions which, in turn., react with sulfate ions in the solution to form lead sulfate which is suspended in the solution. The lead sulfate sludge can be removed by providing a filter in the bath, but the filter requires more manpower for maintenance. If sludge removal is insufficient, it can accumulate on the inner walls of the bath and pipings, obstructing the solution flow. If lead sulfate sludge sticks to the cathode drum, point defects such as pinholes and anomalous deposits would occur in the copper foil. These defects are critically detrimental to the copper foil.
The use of lead electrodes has the drawback that lead can be locally worn out by current concentration and erosion, resulting in a local variation in the cathode-to-anode distance. One solution is to periodically machine the lead anode which leads to not only a lowering of working factor, but also an increased cathode-to-anode distance which in turn, leads to an increased bath voltage and an increased cost. The variation in the cathode-to-anode distance causes a variation in copper foil thickness in a transverse direction.
To prevent occurrence of pinholes and anomalous deposits caused by sulfate sludge and to eliminate a transverse thickness variation of copper foil due to a varying cathode-to-anode distance resulting from lead wear, Japanese Patent Publication (JP-B) No. 56153/1989 discloses an insoluble anode formed of a valve metal substrate such as Ti, Ta, Nb and Zr and coated with a catalytic coating of a platinum group metal or an oxide thereof as the arcuate plate-shaped anode opposed to the cathode drum.
However, this anode is still susceptible to local wear and short-circuiting due to anomalous copper deposition on the cathode drum. Since this anode is a one-piece arcuate plate, the entire anode must be removed and exchanged for repairing such failure. As a result, the operation of maintenance and repair including handling of the anode for mounting in the plating system is cumbersome and time-consuming, the cost of maintenance and the capital equipment are increased, and the plating system has a low working factor.
More undesirably, the use of a one-piece arcuate plate anode is susceptible to concentration of current density at the edges during electric conduction which is known as edge effect. In particular, the edge effect causes current flow to concentrate near the edges of the anode plates which delimit the inlet slit for plating solution, causing local wear of the catalytic coating of the anode plates which results in a length of copper foil varying in thickness in a transverse direction. This foil thickness variation increases during continuous operation and eventually beyond a practically acceptable level, meaning that the anode has a short life. This phenomenon becomes more serious in the manufacture of copper foil which is as thin as 20 .mu.m or less. In fact, the above-referred JP-B 56153/1989 reports a foil thickness variation within 2% in the manufacture of 18-.mu.m thick copper foil. The state-of-the-art is not successful in achieving a foil thickness variation within 1%. Other drawbacks are difficulty to form a coating on a large arcuate substrate and non-uniformity of coating thickness.